We usually use a crystal oscillator for generating clock of a microcontroller… or we can use the internal RC oscillator available with some microcontrollers…. The frequency of this clock is termed as Fosc …

But some microcontrollers have internal PLL, thus by using it we can multiply the crystal or RC oscillator frequency to a large value…. In this case Fosc is the resultant frequency…

While using some peripherals like timers you can divide this Fosc by using prescalar by writing appropriate registers…